Most common skin disorders and complaints comprise a number of different components. Thus, many skin disorders involve (a) hyperproliferation, (b) inflammation and/or (c) dehydration. Hyperproliferation involves a state of abnormally high cell division, which can lead to excess flaking skin. Inflammation involves swelling and redness of the skin, as well as sensations of increased heat, and pain in the skin. Dehydration involves loss of water from the skin and may be due, for example, to damage to the normally waterproof top layer of the skin (epidermis).
Inflammation of the skin (dermatitis) in mammals can result from a number of different etiologies. Dermatitis can be caused by eczema, in particular atopic eczema (atopic dermatitis), disseminated neurodermatitis, flexural eczema, infantile eczema, prurigo diathsique, contact dermatitis, (eg irritant contact dermatitis, allergic contact dermatitis and photocontact dermatitis), xerotic eczema, seborrheic eczema, dyshidrosis, discoid eczema, venous eczema, dermatitis herpetiformis, neurodermatitis and autoeczematisation. Dermatitis can also be caused by skin inflammation resulting from exposure to radiation, in particular exposure to ultraviolet radiation. Other causes of dermatitis includes uremic pruritis and autoimmune diseases, in particular lupus and psoriasis.
Inflammation of the skin causes rashes, redness, skin edema (swelling), itching, blistering, sensations of pain and/or heat and can be unsightly. The itchiness caused by inflammation can lead to scratching. Scratching of skin that is already damaged in some way can easily lead to the barrier of the epidermis being broken, resulting in bleeding, and secondary infection with pathogens. Such secondary infection can require treatment with antibiotics.
It is well known that when treating a skin condition that has a number of different components, i.e. hyperproliferative, inflammatory and/or dehydrative components, a number of different treatments may be used. Thus, in the treatment of psoriasis, for example, an antihyperproliferative may be used to treat the hyperproliferative component of the disease, an anti-inflammatory may be used to treat the inflammatory component and an emollient may be used to treat the dehydrative component.
The most common form of treatment for inflammation of the skin is oral and/or topical steroids. There are, however, drawbacks associated with steroid treatments. Common side effects associated with steroids include stunting of growth, thinning of the skin, muscle loss and osteoporosis.
The present invention relates to new methods for treating skin inflammation, in particular skin inflammation caused by atopic eczema, contact dermatitis, psoriasis or uremic pruritis, in mammals.
Eicosa-8Z,11Z,14Z-trienoic acid (Dihomo-γ-linolenic acid or DGLA) is a commercially available polyunsaturated fatty acid (PUPA). DGLA has the structure shown below.

EP-A-0085579 describes the use of DGLA in combination with antipruritic lithium salts. EP-A-0173478 describes the use of DGLA in combination with anti-inflammatory glucocorticoids. In these applications, treatment with lithium salts and glucocorticoids is supplemented with DGLA, as both lithium salts and glucocorticoids are believed to block release of DGLA from endogenous stores in the body. Advantageously, it has now been discovered that DGLA can be used effectively as a monotherapy.
5-Hydroxy-eicosa-6E,8Z,11Z-trienoic acid (5-HETrE)) is a commercially available PUFA derivative derived from mead acid. 5-HETrE has the structure shown below.

8-Hydroxy-eicosa-9E,11Z,14Z-trienoic acid (8-HETrE) is a commercially available PUFA derivative derived from eicosa-8Z,11Z,14Z-trienoic acid (Dihomo-γ-linolenic acid or DGLA). 8-HETrE has the structure shown below.

15-Hydroxy-eicosa-8Z,11Z,13E-trienoic acid (15-HETrE) is a commercially available PUFA derivative derived from eicosa-8Z,11Z,14Z-trienoic acid (Dihomo-γ-linolenic acid or DGLA). 15-HETrE has the structure shown below.

15-HETrE is known to have antiproliferative properties when applied directly to the skin (Xi, et al; Prostaglandins, Leukotrienes and Essential Fatty Acids (2000) 62(1), 13 to 19).
13-Hydroxy-octadeca-6Z,9Z,11E-γ-trienoic acid (13-HOTrE(γ)) is a commercially available PUFA derivative derived from gamma-linolenic acid (GLA). 13-HOTrE(γ) has the structure shown below.

It has now been surprisingly found that DGLA, 5-HETrE, 8-HETrE, 15-HETrE, 13-HOTrE(γ) and their derivatives are clinically useful in treating skin inflammation, in particular skin inflammation caused by atopic eczema, contact dermatitis, psoriasis or uremic pruritis, by topical administration in mammals. A particular finding of the present invention is that these compounds reduce the level of COX-2 enzymes in the skin when applied topically. The COX-2 family of enzymes have been strongly linked to inflammation and have been found to be present in increased amounts in inflamed tissue.